Probing the Electronic Structure of Substituted Ferrocenes with High-Resolution XANES Spectroscopy
Probing the Electronic Structure of Substituted Ferrocenes with High-Resolution XANES Spectroscopy
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DOI：
10.1002/chem.201200649
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发表时间：
2012-06-01
影响因子：
4.3
通讯作者：
Bauer, Matthias
中科院分区：
文献类型：
--
作者：
Atkins, Andrew J.;Jacob, Christoph R.;Bauer, Matthias
通过X射线光谱方法，特别是通过X射线吸收光谱（XAS）进行的原位研究为催化反应和潜在的分子机制提供了重要的见解。[1]扩展X射线吸收精细结构（EXAFS）区域可以提供金属中心周围几何结构的信息，[2] XAS光谱的XANES（X射线吸收近边结构）区域包含电子结构的信息。在K边XAS实验中，1s电子在激发后到达的第一个未占据态是金属d态。然而，由于这样的1 s → nd跃迁是偶极禁戒的，因此所产生的所谓的前边缘信号（前峰）具有弱强度。[1a]如果考虑分子复合物和催化剂，这些前峰包含有关最低未占分子轨道（LUMO）的详细信息。到目前为止，传统的K边XAS实验的寿命展宽限制了它们探测LUMO态的适用性。另一方面，在L边XAS [3]中，可以使用更强烈的2p → nd跃迁[4]，但由于使用低能辐射，这种技术不可能原位研究催化反应。因此，硬X射线技术具有更好的分辨率的最终dstates比在传统的XAS需要探测催化剂的电子结构原位。高能量分辨率荧光检测X射线吸收光谱（HERFD-XAS）[5]能够减少寿命增宽。在该技术中，通过监测具有小于芯孔的寿命加宽的能量分辨率的选定荧光通道来记录XAS光谱。[6]这样，XAS光谱的XANES区域中的净信号加宽可以显著减小，这使偶极禁戒跃迁的非常弱的信号变尖锐。因此，令人惊讶的是，HERFD-XAS测量尚未应用于克服常规XAS关于涉及分子复合物的过程（如均相催化）的限制。到目前为止，HERFD-XAS和X射线发射研究主要应用于多相催化过程[7]和酶系统。[8]此外，HERFD-XAS已被用于阐明具有铁中心的蛋白质模型的结构。[9]然而，这些配合物在第一配位壳层中已经显示出结构差异。关于HERFD-XAS解决最近邻配位原子以外的结构差异及其对中心金属原子电子结构的影响的能力，我们一无所知。在此，我们希望通过开创性的HERFD-XAS过渡金属络合物研究来弥合这一差距，该研究表明，该技术甚至对与金属中心配位的配体的取代效应敏感，即传统EXAFS分析不可见的效应。作为第一个目标，我们调查的二茂铁化合物的电子结构的变化引起的取代基在Cp（Cp）环。如方案1所示，二茂铁结构基序在整个研究中保持不变。只有取代基发生了变化，这影响了铁中心的电子结构。在这里，我们证明，这些微妙的变化可以探测HERFD-XANES和TD-DFT计算解释。这种取代的二茂铁在催化、[10]可切换自组装单分子层（SAM）[11]和电子通信的制备中发挥着重要作用-[a] AJ阿特金斯，博士CR雅各布功能纳米结构卡尔斯鲁厄理工学院（KIT）Wolfgang-Gaede-Strasse 1a，76131 Karlsruhe.
In situ studies by X-ray spectroscopic methods, especially by X-ray absorption spectroscopy (XAS), have contributed important insights into catalytic reactions and the underlying molecular mechanisms.[1] While the extended X-ray absorption fine structure (EXAFS) region can provide information on the geometric structure around a metal center,[2] the XANES (X-ray absorption near edge structure) region of XAS spectra contains information on the electronic structure. In K-edge XAS experiments, the first unoccupied states reached by the 1s electron after excitation are metal d-states. However, as such a 1 s→ nd transition is dipole forbidden, the resulting so-called pre-edge signals (prepeaks) are of weak intensity.[1a] If molecular complexes and catalysts are considered, these prepeaks contain detailed information about the lowest unoccupied molecular orbitals (LUMO). So far, the lifetime broadening of conventional K-edge XAS experiments limits their applicability for probing the LUMO states. On the other hand, in L-edge XAS [3] a more intense 2p→ nd transition can be used,[4] but because of the use of low-energy radiation, in situ studies of catalytic reactions are not possible with this technique. Therefore, a hard X-ray technique with a better resolution of the final dstates than in conventional XAS is required to probe the electronic structure of catalysts in situ. High-energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS)[5] is capable of reducing the life-time broadening. In this technique, the XAS spectra are recorded by monitoring a selected fluorescence channel with an energy resolution smaller than the life time broadening of the core hole.[6] With this, the net signal broadening in the XANES region of XAS spectra can be significantly reduced, which sharpens the very weak signals of dipole-forbidden transitions. It is therefore surprising that HERFD-XAS measurements have not been applied to overcome the limitations of conventional XAS with respect to processes involving molecular complexes, like in homogeneous catalysis. So far, HERFD-XAS, and also X-ray emission studies, have been mainly applied to heterogeneous catalytic processes [7] and enzymatic systems.[8] Also HERFD-XAS has been used to elucidate the structure of protein models with iron centers.[9] However, these complexes show structural differences already in the first coordination shell. Nothing is known about the power of HERFD-XAS to resolve structural differences beyond the nearest neighbor coordinating atoms and their influence on the electronic structure at the central metal atom. Herein, we want to bridge this gap by pioneering HERFD-XAS studies on transition-metal complexes, which show that this technique is even sensitive for substitution effects at the ligands coordinated to a metal center, that is, effects which are not “visible” to conventional EXAFS analysis. As a first target, we investigate changes in the electronic structure of ferrocene compounds induced by substituents at the cyclopentadienyl (Cp) rings. As indicated in Scheme 1, the ferrocene structural motif remains unchanged throughout the study. Only the substituents are changed, which affects the electronic structure at the iron center. Herein, we demonstrate that these subtle changes can be probed by HERFD-XANES and explained with TD-DFT calculations. Such substituted ferrocenes play an important role in (bimetallic) catalysis,[10] the preparation of switchable self-assembling monolayers (SAMs)[11] and electronic com-[a] AJ Atkins, Dr. CR Jacob Center for Functional Nanostructures Karlsruhe Institute of Technology (KIT) Wolfgang-Gaede-Strasse 1a, 76131 Karlsruhe …